Increasingly, components must be integrated onto supports different from those used to produce them. Components on plastics material substrates or on flexible substrates may be cited, for example. By component is meant any microelectronic, opto-electronic or sensor (for example chemical, mechanical, thermal, biological or biochemical sensor) device that is completely or partly “processed”, such as completely or partly produced.
A layer transfer method can be used to integrate the components onto flexible supports.
There are many other examples of applications in which layer transfer techniques can provide a suitable solution for integrating components or layers onto a support that is a priori unsuited to their production. In the same line of thinking, layer transfer techniques are also very useful when it is required to isolate a thin layer, with or without components, from its original substrate, for example by separating or eliminating the latter. Still in the same line of thinking, turning over a thin layer and transferring it onto another support provides engineers with valuable freedom to design structures that would otherwise be impossible. Sampling and turning over thin films can be used to produce buried structures, for example, such as buried capacitors for dynamic random access memory (DRAM) where, in contradistinction to the usual situation, the capacitors are formed first and then transferred onto another silicon substrate before fabricating the remainder of the circuits on the new substrate. Another example relates to the production of transistor structures referred to as double gate structures. The first gate of the CMOS transistor is produced on a substrate using a conventional technology and then turned over and transferred to a second substrate to produce the second gate and finish the transistor, thereby leaving the first gate buried in the structure (see, for example, K. Suzuki, T. Tanaka, Y. Tosaka, H. Horie and T. Sugii, “High-Speed and Low-Power n+-p+ Double-Gate SOI CMOS”, IEICE Trans. Electron., vol. E78-C, 1995, pp. 360-367).
The requirement to isolate a thin layer from its original substrate is encountered in the field of light-emitting diodes (LED), for example, for instance as reported in the documents W. S Wong et al., Journal of Electronic MATERIALS, page 1409, Vol. 28, No 12, 1999 and I. Pollentier et al., page 1056, SPIE Vol. 1361 Physical Concepts of Materials for Novel Opto-electronic Device Applications I (1990). One of the objects here is to improve the control of extraction of the emitted light. Another object relates to the fact that in this particular example the sapphire substrate used to produce the epitaxial stack is a posteriori bulky, in particular because it is electrically insulative, which prevents making electrical contacts to its rear face. To be able to remove afterwards the sapphire substrate, which was advantageous for the phase of growing the material, would thus appear to be desirable.
An identical situation is encountered in the field of applications to telecommunications and microwaves, for example. In this situation, it is preferable for the components to be finally integrated onto a support having a high resistivity, typically at least several kohms·cm. However, a highly resistive substrate is not necessarily available at the same cost and with the same quality as the standard substrates usually employed. In the case of silicon, 200 and 300 mm silicon wafers of standard resistivity are available, whereas for resistivities greater than 1 kohm·cm, there is very little on offer in the 200 mm size and nothing at all in the 300 mm size. One solution consists in producing the components on standard substrates and then, during the final process steps, transferring a thin layer containing the components onto a glass, quartz, sapphire, etc. insulative substrate.
From a technical point of view, the major benefit of these transfer operations is to decorrelate the properties of the layer in which the components are formed and those of the final support layer, and they are consequently beneficial in many other situations.
There may also be cited situations in which the substrate that is beneficial for the production of the components is excessively costly. In this case, for example that of silicon carbide, offering improved performance (higher temperatures of use, significantly improved maximum powers and frequencies of use, and the like), but whose cost compared to silicon is very high, it would be beneficial to transfer a thin layer of the costly substrate (in this instance silicon carbide) onto the inexpensive substrate (here silicon), and to recover the remainder of the costly substrate for re-use, possibly after a recycling operation. The transfer operation can be carried out before, during or after the production of the components.
The above techniques can also prove beneficial in all fields in which obtaining a thin substrate is important for the final application. Power applications in particular may be cited, whether for reasons associated with the evacuation of heat, which is improved if the substrate is thin, or because in some cases the current must flow through the thickness of the substrates with losses that are to a first approximation proportional to the thickness through which the current passes. Smart card applications for which a thin substrate is required for reasons of flexibility may also be cited. For these applications, the circuits are produced on thick or standard thickness substrates, which has the advantage, firstly, of good mechanical resistance to the various process steps, and, secondly, of conforming to standards with regard to their use on certain production equipment. The final thinning is achieved by separation. This separation can be accompanied by transfer to another support. In some cases the transfer to another support is not indispensable, especially if the final thickness aimed at during thinning is sufficient to produce self-supporting structures.
Various techniques can be used to transfer layers from one support to another. The techniques disclosed in 1985 by T. Hamaguchi et al., Proc. IEDM, 1985, p. 688, may be cited, for example. These techniques are of great benefit since they enable a layer to be transferred from one substrate to another, but they necessarily consume the basic substrate (which is destroyed during the process) and cannot achieve homogeneous transfer of a thin film unless a stop layer is present (i.e. a non-homogeneous layer in the substrate material).
Of the transfer methods known to the person skilled in the art, it is also possible to use one of transferring thin layers of materials that may contain all or part of a microelectronic component. Some of these methods are based on creating a buried weak layer within a material, by introducing one or more gaseous substances. On this subject see the documents U.S. Pat. No. 5,374,564 (or EP-A-533551), U.S. Pat. No. 6,020,252 (or EP-A-807970), FR-A-2767416 (or EP-A-1010198), FR-A-2748850 (or EP-A-902843), and FR-A-2773261 (or EP-A-963598), which disclose such methods.
These methods are generally used with the objective of detaching the whole of a film from an original substrate to transfer it onto a support. The thin film obtained may then contain a portion of the original substrate. These films can serve as active layers for the production of electronic or optical components. They may contain some or all of a component.
These methods in particular enable reuse of the substrate after separation, very little of the substrate being consumed on each cycle. This is because the thickness removed is frequently no more than a few micrometers, whereas substrate thicknesses are typically several hundred micrometers. It is therefore possible to obtain substrates that are similar to substrates that are “demountable” (that is to say detachable) by mechanical stress, in particular in the case of the method disclosed in the document U.S. Pat. No. 6,020,252 (or EP-A-807970). This particular process is based on implantation to form a fragile buried region which is cut at the time of final transfer.
Other methods, based on the “lift-off” principle, also separate a thin layer from the remainder of its original support, again without the latter necessarily being consumed. These methods generally use selective chemical etching of a buried intermediate layer, possibly associated with mechanical forces. This type of method is very widely used to transfer III-V elements to different types of support (see C. Camperi et al., IEEE Transactions on photonics technology, vol. 3, 12 (1991), 1123). As explained in the paper by P. Demeester et al., Semicond. Sci. Technol. 8 (1993), 1124-1135, the transfer, which generally takes place after an epitaxial growth step, can be carried out before or after the production of the components (by “post-processing” or “pre-processing”, respectively).
Of the methods using a (pre-existing) buried layer of lower mechanical strength than the remainder of the substrate to obtain localized separation, the ELTRAN® method may be cited (Japanese Patent Publication Number 07302889). In this case, a stack based on monocrystalline silicon is locally weakened by the formation of a porous region. Another similar situation consists in exploiting the presence of a buried oxide in the case of a silicon on insulator (SOI) structure, however standard the latter may otherwise be (i.e. produced without seeking any particular detachability effect). If the structure is bonded sufficiently strongly to another substrate and a high stress is applied to the structure, localized fracture, preferentially achieved in the oxide, can lead to a cutting effect on the scale of the entire substrate. The document “PHILIPS Journal of Research”, vol. 49, No 1/2, 1995, shows an example of this on pages 53 to 55. Unfortunately, the fracture is difficult to control and necessitates high mechanical stresses to bring it about, which is not free of the risk of breaking of the substrate or damaging the components.
The advantage of such buried fragile layer methods is that they can be used to produce layers based on crystalline silicon (or SiC, InP, AsGa, LiNbO3, LiTaO3, and the like) in a range of thickness from a few tens of angstrom units (A) to a few micrometers (μm), with very good homogeneity. Greater thicknesses can also be achieved.
To fabricate detachable structures for possible subsequent transfer of a layer onto another support or substrate, it is known to the person skilled in the art to control the energies of the bonds between the layer and the substrate, as indicated in the document EP 0702609 A1.
The inventors of this patent are also aware that, to produce a detachable substrate, it is also possible to use methods involving the control of bonding forces existing at the “detachment” surface to assemble together temporarily the thin layer and the substrate from which it is subsequently to be detached. The situation in which the bonding is obtained by molecular adhesion is of particular benefit. Of the categories of assemblies obtained by molecular bonding, silicon on insulator (SOI) substrates produced by these bonding techniques constitute a particularly beneficial category. The category encompasses a number of variants, the principles of which are described in the book “Semiconductor Wafer bonding Science and Technology”, Q.-Y. Tong and U. Gosele, a Wiley Interscience publication, John Wiley & Sons, Inc. Some variants are known as bonded SOI (BSOI) or bond and etch back SOI (BESOI). Apart from bonding involving molecular adhesion, these variants rely on physical removal of the original substrate, by polishing techniques and/or chemical etching techniques. Other variants, described in part heretofore as layer transfer techniques, are additionally based on bonding by molecular adhesion and separation by “cutting” along a region that has been weakened, for example as in the methods described in the documents U.S. Pat. No. 5,374,564 (or EP-A-533551) and U.S. Pat. No. 6,020,252 (or EP-A-807970) (separation along an implanted region) or in the document EP 0925888 (separation by fracture along a buried layer that has been rendered porous). Whatever the exact technique used, the common feature of these variants is their use of molecular bonding, in most of the cases encountered in the literature between two substrates having silicon (Si) or silicon oxide (SiO2) at the surfaces to be brought into contact. Other materials are sometimes encountered (nitrides, silicides, and the like).
If non-detachable SOI structures are to be obtained, the surface preparation operations are intended finally to provide, and often with the aid of annealing carried out after bonding, high bonding energies typically from 1 to 2 J/m2. Conventionally, with standard preparation operations, the bonding energy of the structure is of the order of 100 mJ/m2 at room temperature and 500 mJ/m2 after annealing at 400° C. for 30 minutes, in the case of SiO2/SiO2 bonding (bonding energy determined by the blade method developed by Maszara (see: Maszara et al., J. Appl. Phys., 64 (10), p. 4943, 1988)). When the structure is annealed at a high temperature (1000° C.), the bonding energy can be as high as 2 J/m2 (C. Maleville et al., Semiconductor wafer bonding, Science Technology and Application IV, PV 97-36, 46, The Electrochemical Society Proceedings Series, Pennington, N.J. (1998)). Other forms of preparation prior to bonding exist, for example exposure of the surfaces to be bonded to a plasma (for example an oxygen plasma), and can yield equivalent bonding energies without always necessitating such annealing (Y A, Li and R. W. Bower, Jpn. J: Appl. Phys., vol. 37, p. 737, 1998).